Zarathustra (also given as Zoroaster, Zartosht, Zarathustra Spitama, l. c. 1500-1000 BCE) was the Persian priest-turned-prophet who founded the religion of Zoroastrianism (also given as Mazdayasna “devotion to Mazda”), the first monotheistic religion in the world, whose precepts would come to influence later faiths.
He was a priest of the Early Iranian Religion who received a vision from Ahura Mazda – the chief deity of that faith's pantheon – telling him to correct the error of polytheistic religious understanding and proclaim the existence of only one true god – Ahura Mazda – the Lord of Wisdom.
Zarathustra initially met with harsh resistance to his message until he converted the king Vishtaspa, who then led his people to the new faith. Zarathustra then received messages from Ahura Mazda while he was deep in prayerful meditation which he would repeat to his disciples. These messages came in answer to questions and were memorized by the prophet and his followers as a living scripture which was passed down from generation to generation in the ancient language known as Avestan. The faith was embraced by the Achaemenid Empire (c. 550-330 BCE) and the Parthian Empire (247 BCE-224 CE) who maintained the oral tradition. Under the Parthian Empire, a written record of the conversations between Zarathustra and his God was initiated.
The scriptures were finally committed to writing by the scribes of the Sassanian Empire (224-651 CE) after Zoroastrianism was declared the state religion. The oral tradition in written form became known as the Avesta (also given as Zend Avesta). Zarathustra's vision of a single, all-powerful, all-good God who took a personal interest in the lives and particularly the morality of human beings would inform the later monotheistic faiths of Judaism, Christianity, and Islam.